1. Field of the Invention
The invention relates to a two-stroke engine, in particular, as a drive motor for a hand-held working tool such as a motor chainsaw, a clearing saw, a cut-off saw or the like, comprising a combustion chamber formed in a cylinder which is delimited by a reciprocating piston, wherein the piston drives by a connecting rod a crankshaft rotatably supported in a crankcase. An outlet for removing exhaust gases from the combustion chamber and an intake supplying a fresh mixture into the combustion chamber are provided. The intake forms one end of a storage duct whose other end opens into the crankcase, wherein the storage duct between its ends is connected to a mixture-forming channel of a mixture forming device for a fuel/air mixture. The crankcase comprises a crankcase intake for combustion air and fuel, which is connected with an intake passage of an additional mixture forming device. The crankcase comprises a transfer passage to the combustion chamber which opens at one end by means of a transfer window into the combustion chamber and at the other end is connected with the crankcase.
2. Description of the Related Art
From U.S. Pat. No. 4,253,433 an internal combustion engine is known in which a mixture path of the carburetor opens into a transfer duct which is connected with one end to the combustion chamber approximately opposite the outlet and is open with its other end toward the crankcase. The crankcase has a diaphragm-controlled intake via which, in addition to combustion air, also a proportion of the fuel required for operating the internal combustion engine is to be supplied. In such arrangements, care must be taken that the fuel distribution to the crankcase intake and to the transfer duct is adjusted to the operating conditions of the internal combustion engine, respectively. This is technically complex and requires throttle members in the respective channels, wherein the throttle members are to be coupled position-dependently with one another. In practice, it was found that such internal combustion engines do not operate optimally across the entire operating range. The variable proportions of the supplied air and fuel quantities may cause disturbances particularly during acceleration from idle.
It is an object of the invention to configure a two-stroke engine of the aforementioned kind such that across the entire operating range of the internal combustion engine a good power development and a good acceleration behavior with good exhaust emission values can be reached.
The object is solved according to the invention in that the flow cross-section of the mixture-forming channel leading to the storage duct is smaller by more than one half than the flow cross-section of the intake passage leading to the crankcase intake, and that, during idle of the internal combustion engine, the fuel path opening into the crankcase intake is substantially completely closed and the fuel, required for operation, is supplied substantially exclusively via the mixture-forming channel leading to the storage duct.
This configuration provides that the flow cross-section of the mixture-forming channel leading to the storage duct is designed to be smaller by more than one half than the flow-cross section of the intake passage leading to the crankcase intake. This makes it possible that a sufficiently high flow velocity is provided in the mixture forming channel which, in principle, enables that the required fuel quantity is aspirated. Accordingly, during idle of the internal combustion engine the fuel path leading to the crankcase intake can be substantially completely blocked so that the fuel required for the operation is supplied substantially exclusively via the mixture-forming channel leading to the storage duct. It is thus ensured that during idle operation a sufficiently rich mixture is supplied to the combustion chamber which cannot lead to a significant enleanment within the combustion chamber even for air supply via the crankcase intake. Instead, in the combustion chamber a homogenous ignitable mixture is made available which ensures a smooth idle without disturbances. The mixture distribution which results during idle within the internal combustion engine favors also a powerful acceleration.
Advantageously, not only the fuel required for operation but also the combustion air required for operation is supplied exclusively via the mixture-forming channel leading to the storage duct, at least during idle of the internal combustion engine.
In practice, it was found to be expedient to supply the fuel quantity supplied to the internal combustion engine in a partial load operation and/or in the full load operation in a subordinate proportion of approximately 0% to 35% via the intake passage to the crankcase intake. This proportion serves substantially for lubricating the moving parts and has only a minimal effect on the mixture composition in the combustion chamber.
Important in connection with the constructive configuration of the mixture forming devices to the storage duct and to the crankcase intake is the ratio V of the flow cross-sections relative to one another. It should be in a range of xc2xd to {fraction (1/12)} wherein this refers to the ratio of the flow cross-section of the first mixture-forming channel leading to the storage duct relative to the flow cross-section of the second mixture-forming channel (intake passage) leading to the crankcase intake.
The mixture-forming channel and the intake passage are positioned preferably in a common housing (carburetor housing) and form a double-flow carburetor which can be connected in a simple way to the cylinder socket of the internal combustion engine.
The fuel supply to the first mixture-forming channel and to the intake passage can be realized in many ways. Expediently, the flow cross-section of the first mixture-forming channel can be changed by a mixture-forming channel throttle, wherein then the fuel path into the mixture-forming channel is uncontrolled, i.e., the fuel supply is determined exclusively by the vacuum within the first mixture-forming channel. In the same way, the flow cross-section of the intake passage can be changeable by an intake passage throttle so that the fuel path into the intake passage is also controlled by means of vacuum.
In order to be able to achieve within the different load ranges an adjusted supply of a fuel/air mixture via the mixture-forming channel and fuel and/or combustion air via the intake passage, the throttle arranged within the first mixture-forming channel is expediently coupled position-dependently with the throttle arranged in the intake passage. The coupling is provided such that the first mixture-forming channel is open and the intake passage is closed during idle, while under full load the first mixture-forming channel is closed and the intake passage is open.
It may also be expedient to form the flow cross-section of the first mixture-forming channel and/or that of the intake passage so as to be unchangeably fixed. In this connection it is expedient to configure the fuel path to the first mixture-forming channel or to the intake passage so as to be controllable, in particular, to control it mechanically. For example, a needle valve can be arranged in the fuel path for this purpose.